1. Field of the Invention
This invention generally relates to air filter media. More specifically, the present invention relates to filter media for use in air cleaning operation of clean rooms, liquid crystals and/or semiconductor production devices. The present invention further relates to an air filter pack having the filter medium. The present invention still further relates to an air filter unit having the air filter pack.
2. Background Information
In a clean room or in a semiconductor production device, electronic parts such as semiconductors and liquid crystal display devices are produced in an environment in which the air is treated to capture floating particles. A porous film having polytetrafluoroethylene (hereinafter referred to as xe2x80x9cPTFExe2x80x9d for simplicity) has been used as filter medium for high performance air filters used for cleaning air of such clean rooms, liquid crystals and/or semiconductor production devices. Such porous PTFE film is used usually in the form of a laminated film, with air-permeable supporting members interposing the porous PTFE film from both sides thereof in order to increase strength and easiness in handling.
Collection efficiency and pressure loss are generally known as properties that represent performance of an air filter medium. Specifically, collection efficiency shows the ability of a filter medium to capture floating fine particles in the air. Pressure loss indicates a degree of pressure loss of air that passes through the filter medium. Since porous PTFE film is made from finer fibers than fibers from which a conventional glass filter medium is made, PTFE porous film has high collection efficiency and small pressure loss.
However, when porous PTFE film is laminated by heat fusion with non-woven fabrics, or when the laminate is processed to be shaped into a predetermined shape, fiber structure of the porous PTFE film changes due to pressure from the non-woven fabric. As a result, defects may result in the filter medium, the pressure loss of the filter medium may increase, and/or the collection efficiency thereof may decrease.
In view of the above, there exists a need for filter medium which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
Accordingly, an object of the present invention is to provide an air filter medium having high performance by reducing an increase in its pressure loss and a decrease in its collection efficiency.
Another object of the present invention is to provide a process of manufacturing the high performance air filter medium.
Still another object of the present invention is to provide an air filter pack having the high performance air filter medium.
Yet still another object of the present invention is to provide an air filter unit having the air filter pack.
According to a first aspect of the present invention, an air filter medium is used for air cleaning, and comprises a porous film and a non-woven fabric. The porous film is made of a polytetrafluoroethylene. The non-woven fabric is laminated on at least one side of the porous film. The non-woven fabric on at least one side of the porous film has an apparent density satisfying the following equation:
apparent density (g/cm3) less than 1.5xc3x97(basis weight (g/m2)/1000)+0.11.
It has been found through studies by the applicants of the present invention that hardness of the non-woven fabric used in the air filter medium contributes to the degree of damage that the porous PTFE film receives in laminating the film or other occasions. Further, the applicants have also found that an apparent density of the non-woven fabric is one of factors that determine the hardness of the non-woven fabric.
In view of the above findings, according to the first aspect of the present invention, a non-woven fabric having an apparent density of less than a predetermined value is laminated on at least one side of the porous PTFE film. More specifically, a non-woven fabric softer than the conventional one is laminated on the porous PTFE film. Owing to this structure, the degree of damage that the porous PTFE film receives from the non-woven fabric at the time of lamination is decreased. Also, the degree of increase in pressure loss of the air filter medium and the degree of decrease in collection efficiency thereof are suppressed. As a result, a high performance air filter medium can be obtained.
According to a second aspect of the present invention, in the air filter medium of the first aspect of the present invention, the non-woven fabric on at least one side of the porous film has a compressibility satisfying the following equations:
compressibility (%) less than 0.2xc3x97(basis weight (g/m2))+66, and
compressibility (%)=(d1000 (xcexcm)/d20 (xcexcm))xc3x97100,
where d1000 (xcexcm) is a thickness (xcexcm) under a load of 98 (kPa), and d20 (xcexcm) is a thickness (xcexcm) under a load of 1.96 (kPa).
In the conventional air filter medium, a relatively hard material has been used as a non-woven fabric for the supporting material. As a result, the damage that the porous P film receives was large. It has been found through the studies by the applicants of the present invention that compressibility of the non-woven fabric is one of factors that determine the hardness of the non-woven fabric.
Therefore, in the air filter medium according to the second aspect of the present invention, a non-woven fabric having a compressibility of less than a predetermined value is laminated on at least one side of the porous PTFE film. More specifically, a non-woven fabric softer than the conventional one is laminated on the porous PTFE film. Owing to this structure, the degree of damage that the porous PTFE film receives from the non-woven fabric at the time of lamination is decreased, and the degree of increase in pressure loss of the air filter medium and the degree of decrease in collection efficiency thereof are suppressed. As a result, a high performance air filter medium can be obtained.
According to a third aspect of the present invention, in the air filter medium of the first or second aspect of the present invention, the non-woven fabric on at least one side of the porous film is made of composite fibers having a core-shell structure having a core portion and a shell portion. The core portion is made of a first material with a first melting point. The shell portion is made of a second material with a second melting point. The first melting point is higher than the second melting point.
It has been found through the studies by the applicants of the present invention that, in heat laminating the non-woven fabric made of composite fibers having a core-shell structure on the porous PTFE film, the shell portion is adhered to the porous PTFE film at many fine points, whereby an adhesive layer which is difficult to peel is formed. Also, the air filter medium is prevented from clogging. The applicants have also found that almost no heat shrinkage takes place in the non-woven fabric made of core-shell composite fibers at the time of lamination.
In view of the above findings, according to the third aspect of the present invention, the air filter medium uses the non-woven fabric formed from core-shell composite fibers, and the non-woven fabric has the apparent density or the compressibility having a value less than the predetermined value.
In such an air filter medium, almost no heat shrinkage takes place in the non-woven fabric when the fabric is subjected to heat lamination, and as a result, the degree of damage that the porous PTFE film receives from the non-woven fabric at the time of lamination is suppressed effectively.
According to fourth to sixth aspects of the present invention, a process of producing an air filter medium is a production process of the air filter medium of the first, second and third aspects of the present invention. The process of producing an air filter medium includes a first step and a second step. The first step is for producing a porous film, and the second step is for laminating a non-woven fabric on the porous film on a heating roll, which has undergone a non-adhesive treatment.
In producing the air filter medium, the step of heat laminating the non-woven fabric on the porous PTFE film is generally carried out such that the porous PTFE film and the non-woven fabric are bonded to each other by heat fusion through heat from the heating roll.
However, when a side of the non-woven fabric which is in contact with the heating roll is heated by the heating roll, the side becomes viscous. As a result, adhesive force is generated between the heating roll and the non-woven fabric and peeling takes place between the non-woven fabric and the porous PTFE film. As a result, the porous PTFE film is damaged to cause defects in the air filter medium and deterioration of performance thereof.
In view of the above, according to the fourth to sixth aspects of the present invention, the production process is conducted such that the non-woven fabric is laminated on the porous film on a heating roll which has been subjected to non-adhesive treatment. This structure makes it possible to suppress generation of adhesive force between the non-woven fabric and the heating roll in heat laminating the non-woven fabric on the porous PTFE film, thereby suppressing more effectively the defects in the air filter medium, and the degrees of increase in pressure loss and decrease in collection efficiency of the filter medium. As a result, a high performance air filter medium can be obtained.
Examples of the heating roll which has been subjected to non-adhesive treatment include: rolls obtained by covering the heating roll with a non-adhesive sheet composed of a sheet-like reinforced fiber material layer impregnated with an organic polymeric material that is non-adhesive; rolls obtained by coating the roll surface with an organic polymeric material that is non-adhesive; and rolls the surface of which is formed from rubbers containing non-adhesive polymeric materials.
There is no particular limitation put to the organic polymeric material that is non-adhesive. Examples of the non-adhesive, organic polymeric material include synthetic resins such as fluorine resin, silicone resin or olefin resin, rubbers containing these synthetic resins, and elastomers containing these synthetic resins, and the like.
Examples of the reinforced fiber material include polyester type synthetic fibers, nylon type synthetic fibers, aramid fibers, glass fibers, carbon fibers and ceramic fibers, or the like.
According to a seventh aspect of the present invention, in the process of producing a filter medium according to the fourth aspect of the present invention, the non-adhesive treatment includes covering with a non-adhesive sheet a surface of the heating roll; and the non-adhesive sheet is formed by impregnating a sheet made of a glass fiber material with a fluorine resin material.
It has been found through the studies by the applicants of the present invention that, when the heating roll is covered with the non-adhesive sheet, almost no adhesive force is generated between the non-woven fabric and the heating roll, thus making the non-woven fabric excellent in terms of easiness in peeling.
Thus, according to the seventh aspect of the present invention, the production process of the air filter medium uses the heating roll covered with the above-described non-adhesive sheet. Owing to this structure, the non-woven fabric can easily be peeled from the heating roll after heat lamination. As a result, the degree of damage that the porous PTFE film receives from the non-woven fabric can be minimized. Further, this structure facilitates cleaning of the heating roll and replacement of the non-adhesive sheet.
According to eighth to tenth aspects of the present invention, an air filter pack for air filters includes a filter medium of the first to third aspects of the present invention having a predetermined shape.
The air filter medium is mainly used as a component of an air filter unit that will be discussed later. For this reason, the filter medium is processed into a predetermined shape. For example, the filter medium is folded in an accordion fashion with each fold having a given width. The air filter medium which is processed into a predetermined shape is referred to as an air filter pack.
The air filter pack uses as the non-woven fabric a material having an apparent density less than the predetermined value, i.e., a softer material compared with the conventional ones. This decreases damage that the porous PTFE film receives when the filter medium is processed into a predetermined shape. Thus, formation of defects in the air filter pack can be suppressed.
According to a tenth aspect of the present invention, in the air filter pack of the eight aspect of the invention, the non-woven fabric on at least one side of the porous film is made of composite fibers having a core-shell structure with a core portion and a shell portion. The core portion is made of a first material with a first melting point, and the shell portion is made of a second material with a second melting point. The first melting point is higher than the second melting point.
According to eleventh to thirteenth aspects of the present invention, an air filter unit for air cleaning comprises a frame and an air filter pack having a filter medium of the first to third aspects of the present invention with a predetermined shape that is coupled to the frame.
The air filter unit uses a material having an apparent density less than the predetermined value, i.e., a softer material compared with the conventional ones, as the non-woven fabric used in the air filter medium. This decreases damage that the porous PTFE film receives from the non-woven fabric when forming the air filter unit. Thus, formation of defects in the air filter unit can be suppressed.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.